1. Field of the Invention
This invention relates to certain dyes in photographic elements, compositions, and processes and preferably in photothermographic elements, compositions, and processes which do not require the presence of a silver halide to provide a developable latent image using a reasonably short exposure time. In one of its aspects, it relates to photographic elements containing these dyes. In another of its aspects, it relates to a photographic composition containing the described dyes. A further aspect relates to a method of developing a latent image in the described photothermographic element by heating the element. A still further aspect relates to the method of physically developing photographic elements containing the dyes. The photographic elements, compositions and processes of the invention are useful in the graphic arts.
2. Description of the Prior Art
Photographic materials are known containing silver halide which can be imagewise exposed and processed in a physical developer or, if photothermographic, can be heat developed to form photographic line and halftone reproductions. Such photographic materials, after exposure, can be developed by conventional physical developers or, if photothermographic, can be heated overall to provide a developed image in the absence of separate processing solutions. If a photothermographic process is to be used, reducing agents are necessary in the imaging composition. Reducing agents which have been employed with silver halide in photothermographic materials are described, for example, in Belgian Pat. No. 765,452, issued May 28, 1971; Belgian Pat. No. 765,602, issued May 29, 1971; Belgian Pat. No. 765,601, issued May 28, 1971; Belgian Pat. No. 766,590, issued June 15, 1971; Belgian Pat. No. 766,589, issued June 15, 1971; Belgian Pat. No. 722,371, issued Oct. 15, 1971; U.S. Pat. No. 3,152,904 of Sorenson et al., issued Oct. 13, 1964; U.S. Pat. No. 3,152,903 of Sorenson et al., issued Oct. 13, 1964; U.S. Pat. No. 3,457,075 of Morgan et al, issued July 22, 1969; U.S. Pat. No. 3,392,020 of Yutzy et al, issued July 9, 1968 and British Pat. specification No. 1,161,777 published Aug. 20, 1969.
The use of photographic silver halide to achieve high contrast films is undesirable. The high expense and shortage of silver has resulted in a continuing search for substitutes. However, at this time no material has been found to provide the high contrast provided by photographic silver halide at reasonable exposure times.
One photothermographic material without silver halide is described in Lokken U.S. Pat. No. 3,409,438. This material contains certain reducing agent precursors to form an image after imagewise exposure and heat development. These photothermographic elements, however, are not capable of forming high contrast images after being exposed for only short periods of time.
Use of certain nitrobenzylidene dyes as impregnates for support materials for photograhic elements containing silver halide is described in Belgian Pat. No. 788,279 published Feb. 28, 1973. A non-silver halide-containing photothermographic element has not been found that is capable of being exposed for a short time and heat developed to a high-contrast image.
Accordingly, there has been a continuing need to provide improved photographic and photothermographic elements, compositions and processes that do not contain silver halide to provide high contrast images with reduced exposure time.